This invention relates generally to radiological well logging methods and apparatus for investigating the subsurface earth formations traversed by a borehole and, more specifically, relates to an improved system for high frequency pulsed neutron gamma ray logging methods and apparatus wherein gamma rays resulting from neutron inelastic scattering and thermal neutron capture are selectively detected and the spectral distribution of the gamma rays is determined.
The selective detection of characteristic gamma rays emanating from earth elements undergoing neutron bombardment is appreciated by those skilled in the art as a method for identifying such elements. More specifically, the detection of gamma rays from carbon, oxygen, silicon, calcium and certain other elements enables the identification of the general rock types in formations traversed by boreholes and the determination of the presence or absence of hydrocarbons within their pore spaces.
Various radioactivity well logging techniques have been used in the prior art to provide lithology information and infer the presence of fluid in a subsurface formation. Basically, these techniques involve irradiating the formation of interest with neutrons and then determining the effect of formation constituents on the neutrons by detecting scattered neutrons or the secondary gamma rays which return to the borehole from the irradiated formation.
One important measurement that can be made is the determination of the ratio of the carbon and oxygen gamma rays resulting from inelastic collisions of fast neutrons with formation elements. This C/O ratio is useful for determining the extent of hydrocarbon saturation in formation fluids if the relative amounts of silicates and carbonates in formation rocks can be determined.
Accordingly, it has been proposed in the prior art to measure a portion of the inelastic and/or capture gamma ray spectra resulting from neutron irradiation of the elements surrounding the borehole, and to derive an indication of the silicon (Si) and calcium (Ca) content of the constituent rocks. In particular, it has been proposed to derive a ratio of the number of counts from selected portions of the spectra to obtain a ratio proportional to the relative amounts of Si and Ca present in formations traversed by a borehole. Deriving such a ratio provides a relative indication of the amounts of these elements present in formation rocks, information that is essential for reliable interpretation of C/O measurements.
Previously, the prior art measurement of Si/Ca ratios has suffered from two major problems. First, the measurement using gamma rays from the capture of thermal neutrons in the formation constituents is highly sensitive to formation and borehole water salinity, due to the large cross section for thermal neutron capture of the chlorine (Cl) nucleus. The effect of the Cl is to depress the Si/Ca ratio and cause high salinity, high porosity water sands to give approximately the same response as limestone. The energy intervals used in the method of the prior art were selected to minimize this chlorine dependency, but were effective only in low to medium porosities and moderate water salinities. This chlorine dependence complicates the analysis of complex lithologies.
One solution to the salinity problem that has been proposed in the prior art, for example, in U.S. Pat. No. 3,780,301, is to ratio the Si and Ca gamma rays resulting from inelastic collisions of energetic neutrons with formation elements. This measurement is substantially independent of water salinity but the ratios so derived show a relatively small difference between sandstone and limestone, again making the identification of mixed lithologies difficult.
Accordingly, it is an object of the present invention to provide new and improved methods and apparatus for identifying constituent elements in a formation traversed by a borehole and, more specifically, to provide improved methods and apparatus for deriving the ratio of Si and Ca gamma rays resulting from the capture of thermal neutrons.
It is also an object of the present invention to derive such a ratio in a manner that provides clear delineation between sandstone and limestone. Further, it is an object of the present invention to provide new and improved methods and apparatus for deriving a ratio of gamma rays, induced by the capture of thermal neutrons in an earth formation, which is substantially independent of water salinity.
The objects of the invention are accomplished, generally, by method and apparatus which generates first electrical signals functionally related to the summed gamma rays resulting from the captured thermal neutrons within a pair of non-contiguous energy windows in the thermal neutron capture spectra to provide an indication of the calcium and which generates second electrical signals functionally related to the gamma rays resulting from the captured thermal neutrons within a third energy window in the thermal neutron capture spectra intermediate the two calcium windows to provide an indication of the silicon.